Solid electrolytic capacitors made of a valve-acting metal such as tantalum, niobium or the like have been used for electronic devices requiring high capacity and compactness.
FIG. 42 is a sectional view showing an example of a conventional solid electrolytic capacitor. Referring to FIG. 42, a solid electrolytic capacitor 90 is of a surface mounting type and includes a resin package 91, a solid electrolytic capacitor chip 92, an anode lead 93 and a cathode lead 94. The resin package 91 is made of a thermosetting resin such as epoxy resin or the like and covers the entire surface of the solid electrolytic capacitor chip 92. The resin package 91 also covers the anode lead 93 in a manner to expose an anode lead portion 931 of the anode lead 93 and the cathode lead 94 in a manner to expose a cathode lead portion 941 of the cathode lead 94. Such a solid electrolytic capacitor 90 is manufactured in substantially the same way as the electronic components of a resin package type semiconductor and so on. That is, after the solid electrolytic capacitor chip 92 is fixed between lead frames for manufacture, a mold surrounding the solid electrolytic capacitor chip 92 with resin is formed. Thereafter, the lead frames are machined into a predetermined shape while removing unnecessary portions of the lead frames.
The solid electrolytic capacitor chip 92 is, for example, a dry tantalum capacitor chip. The solid electrolytic capacitor chip 92 includes an anode wire 921, a cathode metal film 922 and a porous sintered body 923. The anode wire 921 has an elongated shape with a predetermined diameter and is made of tantalum, as shown in FIG. 42. A proximal end 921a (the right end in FIG. 42) of the anode wire 921 is buried in the porous sintered body 923. A distal end 921b (the left end in FIG. 42) of the anode wire 921 is connected to the anode lead 93, as shown in FIG. 42. The cathode metal film 922 covers the porous sintered body 923 in a manner to expose only the left end of the porous sintered body 923 in FIG. 42. The cathode metal film 922 is connected to the cathode lead 94 via a silver paste 95. The porous sintered body 923 is made by sintering tantalum powder pressed into a predetermined shape under a vacuum atmosphere and forming an oxide film on the resulting sintered body. Pores of the porous sintered body 923 are filled with semiconductors such as manganese dioxide and the like.
Connection between the distal end 921b of the anode wire 921 and the anode lead 93 is made by welding using, for example, a laser. Specifically, the distal end 921b of the anode wire 921 and the anode lead 93 are brought into contact with each other, and then a laser is irradiated on a contact portion therebetween. The laser-heated contact portion between the distal end 921b of the anode wire 921 and the anode lead 93 is melted and then cooled to be welded together.
The solid electrolytic capacitor 90 is assembled in, for example, an electronic device. The solid electrolytic capacitor 90 requires a low equivalent series resistance (ESR) in order to lower power consumption of the electronic device. A method of lowering the ESR of the solid electrolytic capacitor 90 may be increasing the diameter of the anode wire 921 to thereby increase the contact area between the anode wire 921 and the porous sintered body 923.
However, increasing the diameter of the anode wire 921 may cause a problem in that the amount of heat required to weld the anode wire 921 with a laser increases with an increase of the diameter of the anode wire 921. Accordingly, heat transferred to the anode wire 921 during the welding increases as much as the diameter increases, which results in an increase in the amount of heat transferred to the porous sintered body 923 via the anode wire 921. The increase in the amount of heat transferred to the porous sintered body 923 may lead to damage of the oxide film formed on the porous sintered body 923. The damage to the oxide film may cause a current leakage between the anode wire 921 and the cathode metal film 922 when using the solid electrolytic capacitor 90.
FIG. 43 is a perspective view showing an example of another conventional solid electrolytic capacitor. Referring to FIG. 43, a solid electrolytic capacitor 90 includes a cubic resin package 91, a solid electrolytic capacitor chip 92, an anode wire 921, an anode lead 93, a lead side connector 95 and a cathode lead 94. The resin package 91 is made of a thermosetting resin such as epoxy resin or the like and covers the entire surface of the solid electrolytic capacitor chip 92, the anode wire 921 and the lead side connector 95 and a partial surface of the anode lead 93 and the cathode lead 94. Portions of the anode lead 93 and the cathode lead 94 are exposed from the resin package 91 and these exposed portions act as terminals of the solid electrolytic capacitor 90. The solid electrolytic capacitor 90 is mounded on, for example, a printed circuit board. The above-mentioned terminals are used to mount the solid electrolytic capacitor 90 on the printed circuit board.
As shown in FIG. 43, the solid electrolytic capacitor chip 92 has a cubic shape and the anode wire 921 extends and projects from one surface of the chip 92 in the longitudinal direction. The lead side connector 95 is disposed at one side of the solid electrolytic capacitor chip 92 in the longitudinal direction and welded to the anode wire 921. With this arrangement, the resin package 91 is formed to be longer than the solid electrolytic capacitor chip 92 in the longitudinal direction by as much as the lengths of the anode wire 921 and lead side connector 95.
Recently, there has been an increasing need for high capacity for the solid electrolytic capacitor 90. In general, the solid electrolytic capacitor 90 is required to have a large volume in order to obtain a high capacity. In this case, the resin package 91 covering the solid electrolytic capacitor 90 is also required to have a large volume, which results in an increase in size of the solid electrolytic capacitor 90.
On the other hand, the printed circuit board on which the solid electrolytic capacitor 90 is mounted becomes more compact with the miniaturization of electronic components. Accordingly, there is an inevitable need for the miniaturization of the solid electrolytic capacitor 90. However, as described above, in order to achieve the high capacity of the solid electrolytic capacitor 90, it is difficult to avoid increasing the size of the solid electrolytic capacitor 90 and achieving miniaturization of the solid electrolytic capacitor 90.
In the related art, there has been proposed a solid electrolytic capacitor using manganese dioxide as a solid electrolytic layer. An electrode substrate of the proposed solid electrolytic capacitor includes a base material made of nickel or a nickel alloy, for example, a 42 alloy (including nickel of 42% and iron of 58%) or a Cu—Ni—Zn alloy (also called a German alloy), a copper underlying layer plated on the base material, and a tin or solder layer plated on the copper underlying layer.
Using the above-mentioned material as a plating material is a result of considering connection stability of a conductive bonding member for connecting a solid electrolytic capacitor chip and an electrode substrate and solder adhesiveness (i.e., solder wettability) during the process of mounting chip components.
In addition, in a solid electrolytic capacitor of a type using a conductive polymer as a solid electrolytic layer, a frame base material made of a copper-based metal, silver plating, gold plating or palladium plating may be used to prevent the electrical characteristics of a solid electrolytic capacitor chip from being damaged.
Specifically, the chip-shaped solid electrolytic capacitor of the conductive polymer type places an emphasis on high conductivity, as described above, and, in many cases, uses a 3-layered plating structure including nickel/palladium/gold.
There has also been proposed a solid electrolytic capacitor of a conductive polymer type which prevents deterioration of ESR (Equivalent Series Resistance) characteristics while avoiding the use of expensive metal such as gold or palladium in order to reduce production costs.
However, the above-mentioned conventional capacitor has a problem in that a solid electrolytic capacitor chip may be damaged by heat transferred thereto from an electrode substrate during a solder mounting process or a reflow process, for example. To overcome this problem, the use of a metal having low thermal conductivity may be considered. However, selectable metals are limited when considering compatibility with materials contacting the metal or the conductivity of the metal.
FIG. 44 is a sectional view showing an example of another conventional solid electrolytic capacitor. As shown in FIG. 44, the solid electrolytic capacitor has a structure in which a solid electrolytic capacitor chip 92, a lead side connector 95 and an electrode substrate 96 are assembled together. The solid electrolytic capacitor chip 92 has a structure in which a porous sintered body 923, a dielectric layer 13, a solid electrolytic layer 14 and a cathode lead-out layer 15 are sequentially formed, and includes an anode wire 921. The cathode lead-out layer 15 is bonded to the electrode substrate 96 via a bonding member 17.
The electrically functioning part of the solid electrolytic capacitor in FIG. 44 is mainly divided into the solid electrolytic capacitor chip 92 and the electrode substrate 96. The base material of the electrode substrate 96 is required to have properties such as, for example, mechanical strength, heat resistance (thermal peeling resistance), thermal conductivity and soldering wettability.
Meanwhile, there has also been proposed a technique for alleviating stress distortion due to thermal stress in a resin sealing type semiconductor device by forming a die pad loading semiconductor chips into a multi-layered structure, in which the thermal expansion coefficient of a first layer is larger than that of a second layer disposed above the first layer.